1. Field of the Invention
The invention relates to a method and system for regulating engine idle speed of an internal combustion engine equipped with an electronic throttle.
2. Background of the Invention
In engines equipped with electronic throttles, airflow to the engine is controlled based on demanded engine torque, which is determined from accelerator pedal position. This torque-based type of control is suitable for operating conditions for which the operator is demanding a non-negligible torque. However, at idle, in which the driver is demanding no torque to be delivered to the wheels of the vehicle, the desire is to maintain a constant engine speed. Commonly, airflow is feedback, controlled to provide a desired constant engine speed during idle. The inventors of the present invention have recognized a problem in combining control based on airflow at idle and torque based control at higher torque conditions. Specifically, the inventors have recognized that engine speed my deviate from the desired value due to a torque bump when traversing between the two engine control modes.
Additionally, the inventors have recognized airflow-based control at idle leads to degraded control over engine speed during a transition in operating mode of a variable displacement engine. A variable displacement engine is one in which some of the engine cylinders are deactivated at low torque causing the engine to deliver higher fuel economy than using all engine cylinders to deliver the desired torque. The problem is in maintaining constant idle speed when a transition in the number of activated cylinders occurs.
The inventors of the present invention have controlled two actuators, e.g., spark and throttle, controlling both based on a single idle torque. Idle speed control was degraded using this method because the two actuators operating on the same request to alter idle torque interfere with each other thereby failing to provide sufficient engine speed regulation.
The present invention addresses shortcomings discussed above by providing a method and system for regulating idle speed based on a power requirement.
Under the invention, a method for regulating idle speed of an engine includes determining a target engine idle speed based on an engine operating condition; determining a power requirement based on the target engine idle speed; determining actual engine speed; controlling a first engine actuator (e.g., a slower actuator) based on the power requirement and the target engine idle speed; and controlling a second engine actuator (e.g., a faster actuator) based on the power requirement and the actual engine speed.
In one embodiment of the invention, the first engine actuator is a slow engine actuator that may require multiple engine cycles to effect a change in engine speed. Because of its relatively slower ability to respond, it is controlled based on the target speed desired. The second engine actuator is a fast engine actuator that is capable of affecting engine by, for example, the next combustion event. Because of its relatively faster ability to respond, the second actuator can respond to situations that make the actual engine speed change. Examples of slow engine actuators include throttle valve actuators and valve timing actuators. Examples of fast engine actuators include ignition actuators and fuel actuators.
The method may further include adjusting the power requirement based on deviation of the actual engine speed from the target engine idle speed to obtain an adjusted power requirement. In addition, the method may include determining a desired power reserve, and adjusting the adjusted power requirement based on the desired power reserve to obtain a first adjusted power requirement. The step of controlling a first engine actuator may then comprise controlling the first engine actuator based on the first adjusted power requirement and the target engine idle speed.
The method may be applied to an engine which is a multi-cylinder, variable displacement engine capable of deactivating one or more of said cylinders. The method may further include controlling the first actuator, preferably a throttle valve, based on the number of deactivated cylinders and controlling the second actuator, preferably a spark advance timing, also based on the number of deactivated cylinders.
The method may further include determining a desired power ratio based on the desired power reserve, determining an actual power ratio based on engine operating conditions, and adjusting the adjusted power requirement based on the difference between the desired power ratio and the actual power ratio to obtain a second adjusted power requirement. Controlling a second engine actuator may then be based on the second adjusted power requirement and the actual engine speed.
Further under the invention, a system for regulating engine idle speed of an engine includes an operating condition sensor for sensing an engine operating condition, and an engine speed sensor for sensing actual engine speed. The system further includes an electronic control unit in electrical communication with the operating condition sensor and the engine speed sensor, and first and second engine actuators in electrical communication with the electronic control unit. The electronic control unit includes instructions for determining a target engine idle speed based on the engine operating condition, instructions for determining a power requirement based on the target engine idle speed, instructions for controlling the first engine actuator based on the power requirement and the target engine idle speed, and instructions for controlling the second engine actuator based on the power requirement and the actual engine speed.
According to the present invention, idle control is based on torques computed for first and second actuators. Since control outside of idle is also based on torque, a transition between idle and non-idle is facilitated by the present invention. The inventors of the present invention have recognized an advantage of the present invention is that a smoother transition is possible between the two operating regimes. Specifically, the transition occurs without incurring a speed deviation or discontinuity, which would be undesirable to the operator of the vehicle.
The present invention also provides smooth transitions among operating modes in a variable displacement engine (VDE) during idle. A VDE disables some of the engine""s cylinders when demanded engine torque is low to provide increased fuel economy. A VDE may also operate with some of the cylinders disabled at idle to obtain fuel savings. However, there are situations in which operation of all cylinders may be requested at idle: during cold weather operation to heat the engine and after-treatment system and to provide smooth operation; during performance of an engine diagnostic operation, such as an emission control system evaluation; during carbon canister vapor purge; and others. As a result, a transition between partial and full cylinder operation may occur during idle. The inventors of the present invention have recognized that by regulating engine speed during idle according to the present invention, i.e., based on controlling the first and second actuators on first and second torques, respectively, and further basing the control of the actuators on the number of deactivated cylinders, a transition between partial and full cylinder operation, and vice versa, of the VDE can occur without a speed flare because the idle speed controller is still in control during the transition.
The inventors have also recognized another advantage of the present invention by basing the torque calculation for controlling the first actuator on the desired or target idle speed and basing the torque calculation for controlling the second actuator on the actual idle speed that idle speed regulation is more robust than if both actuators were controlled based on the same torque.